Well, it obviously depends on your other inhabitants.
As far as the space goes, a group of 5 or 6 should be fine and that's a nice number for them. They only grow to about 3 inches and so far from being territorial are happiest when they are all crammed into a tiny space together or sharing the same piece of food. As long as they have room to swim about, some hiding places and the water stats are good, they should be fine.
If you want more room for other fish, you can keep a group of 4 or even 3 (but not less, they do need to school).
If you want ONLY corys I suppose you could up numbers a bit, but they are very good community fish so you might as well have something for the upper layers.
